Engines may be configured with direct fuel injectors that inject fuel directly into a combustion cylinder (direct injection), and/or with port fuel injectors that inject fuel into a cylinder port (port fuel injection). Multi-fuel engine systems can use both port and direct injection with different fuel types provided to the different injectors. For example, direct injection of ethanol fuel may be used with port injection of gasoline fuel. Therein, the direct injection of the alcohol fuel may take advantage of the increased charge cooling effects of the alcohol fuel's higher heat of vaporization and increased octane. This helps to address knock limitations, especially under boosted conditions. Further, the port injection of the gasoline fuel may take advantage of the higher power output of the gasoline fuel.
Direct injected engines, however, also generate more particulate matter emissions (or soot) due to diffuse flame propagation wherein fuel may not adequately mix with air prior to combustion. Since direct injection, by nature, is a relatively late fuel injection, there may be insufficient time for mixing of the injected fuel with air in the cylinder. Similarly, the injected fuel may encounter less turbulence when air is flowing past the valves. Consequently, there may be pockets of rich combustion that may generate soot locally, degrading exhaust emissions.
One approach for reducing particular matter emissions generated by direct injection of fuel is shown by Bidner et al. in US2011/0162620. Therein, an amount of fuel injected into the cylinder, between the direct injector and the port fuel injector, is adjusted based on the amount of particulate matter (PM) produced by the engine. For example, as a soot load increases, a fuel injection amount from the direct injector is decreased while a fuel injection amount from the port injector is correspondingly increased.
However, the inventors herein have identified potential issues with such an approach. In multi-fuel engine systems, where the fuel that is port injected is different (e.g., with a different alcohol content or a different fuel volatility) from the fuel that is direct injected, the shift towards more usage of the direct injected fuel may not sufficiently reduce PM emissions to meet the mandated low PM emission standards. For example, during selected engine operating conditions, the direct injection may be performed too late such that by the time the direct injection occurs, the emitted PMs are higher than emission standards. As another example, the higher soot load generated by the port injected fuel may eclipse the lower soot load generated by the direct injected fuel, obscuring the PM reduction resulting from the direct injection. The problem may be further exacerbated in multi-fuel engine systems due to the varying availability of the different fuels. For example, due to reduced availability of a first fuel (with a higher alcohol content or a higher fuel volatility) that is coupled to the direct injector, direct injection of the first fuel may be reduced and port injection of a second fuel (with a lower alcohol content or a lower fuel volatility) may be increased. As another example, increased port injection may be enabled for alternate reasons such as increased residence time of a fuel in the corresponding fuel tank. Further still, the fuel volatility may have interactions with the injector type. For example, port fuel injection can have problems with fuel volatility while direct injection is relatively insensitive to fuel volatility. As a result, fuels having higher volatility (e.g., fuels with higher T50 values) can greatly degrade cold port injected fuel combustion but have a much smaller effect on direct injected fuel combustion. As such, this can degrade combustion stability and increase potential for engine misfire. Overall, fuel economy and cold-start exhaust emissions may be degraded.
The above issues may be at least partly addressed by a method for an engine comprising: during a first combustion event since engine start, port injecting a first amount of a first fuel during a closed intake valve event; and direct injecting a second amount of a second fuel over multiple injections of the first combustion event, the first fuel having a lower alcohol content than the second fuel. In an alternate embodiment, the first fuel may have a lower fuel volatility than the second fuel. In this way, benefits from different fuel types as well as from a fuel injection split between port injection of a first fuel and (single or multiple) direct injection(s) of a second fuel can be synergized.
In one example, during an engine start, an engine control system may inject a first fuel having a lower alcohol content, or a lower fuel volatility (such as gasoline) for the first cylinder combustion event, as a first port injection delivered during a closed intake valve event (e.g., during an exhaust stroke of a previous cylinder). Further, a second fuel having a higher alcohol content, or a high fuel volatility (for example, a gasoline-ethanol blend such as E85) may be injected, on the first cylinder combustion event, as a second direct injection, the direct injected fuel delivered over multiple direct injections. For example, the direct injected second fuel may be delivered with at least one injection during an intake stroke, and at least one injection during a compression stroke. This may constitute a first injection profile. The first injection profile may be continued during cranking for a number of combustion events, based on the cylinder event number, until a threshold cylinder event number is reached. The number of combustion events over which the first injection profile is maintained may be based at least on the alcohol content or fuel volatility of the first and/or second fuel. For example, as the alcohol content of the second fuel increases, the number of combustion events may be increased (e.g., up to cylinder event number 24). Alternatively, as the fuel volatility of the second fuel increases, the number of combustion events may be increased. Further, the first ratio of port injected first fuel to direct injected second fuel may also be increased as the alcohol content of the second fuel relative to the alcohol content of the first fuel increases.
By injecting a first fuel with less alcohol content, or less volatility, as a port injection and a second fuel with more alcohol content, or more volatility, as a direct injection, an exhaust catalyst temperature can be rapidly increased to a light-off temperature, improving engine performance at engine cold-starts. By also splitting the direct injection so that some of the direct injected higher alcohol fuel is injected during the intake stroke and the remaining part of the fuel is injected during the compression stroke, the catalyst light-off temperature can be attained without raising exhaust particulate matter (PM) emissions and degrading engine combustion stability. At the same time fuel economy is improved. After a target cylinder combustion event number is reached, the injection profile may be transitioned to a second injection profile that is configured for engine idle speed control. The second injection profile may include, for example, only port injection of the first fuel, only direct injection of the second fuel (e.g., only in the intake stroke or only in the compression stroke), and/or a split ratio that is different from the split ratio of the first injection profile, with a higher percentage of second fuel direct injected. In still further embodiments, the injection profile during an engine cold start may vary based on the engine temperature at the cold start (e.g., based on whether it is a nominal temperature engine cold start or a very cold temperature engine cold start).
In this way, by using a split injection profile that splits fuel injection of different fuels between a port injection and multiple direct injections based on the properties of each available fuel, each fuel can be leveraged to reduce an activation time for an exhaust catalyst while also reducing gaseous and particulate matter emissions. At the same time, a higher amount of spark retard can be tolerated without affecting combustion stability. As such, this allows fuel injection to be optimized so as to enable the benefits of a fuel injection split between a port injection of a first fuel and direct injection of a second, different fuel to be synergized with the benefits of multiple direct injections, and be further synergized to take advantage of the different alcohol content of the different fuels. Overall, engine performance and exhaust emissions are improved, and further, fuel economy is also improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.